Degradable materials, such as poly(vinyl chloride), presently are extruded through annular dies to form tubes, film or parisons for subsequent blow molding. Both single and multiple screw extruders are employed. In the single screw extruder, the material is melted on the screw, intensively mixed and pumped through the die orifice at a temperature of 410.degree.-430.degree. F. In the multiple screw extruder, fusion is delayed until the material enters the last few flights of the screws. The multiple screw extruder utilizes deep-flighted screws and lower shear rates resulting in less mechanical energy input to the plastic, whereby the stock temperature after passing through the die is, typically, 380.degree. F. Due to the lower temperatures of the material on the screw and the lower final stock temperature, it is possible to employ less stabilizer in multiple screw poly(vinyl chloride) formulations with appreciable cost savings.
The annulus for plastic extrusion is generally formed between an inner mandrel supported in concentric relation to an outer die member by a plurality of streamlined spider legs. The plastic stream must flow around these spider legs and then weld downstream in the annular area. These spider legs present an area for stagnation and subsequent degradation of degradable materials such as poly(vinyl chloride). The problem is most severe on single screw extruders, because of the high stock temperatures employed, and is less pronounced on multiple screw extruders. However, on both types of equipment, the length of run often is limited by material degradation on the spider legs.
It is known that high molecular weight poly(vinyl chloride) provides superior impact strength, higher elastic modulus and higher tensile strength. However, such material has a higher melt viscosity which results in poor processability on either single or multiple screw extruders. This is due to non-uniform shearing patterns found in both single and multiple screw extruders, resulting in differences in degree of mixing and final stock temperature within the melt.
An extrusion die made in accordance with this invention is particularly adapted for processing high molecular weight resins to produce smooth, uniform extrudates having fully developed physical properties.